The invention relates to the wear/erosion seen on boiler tubes located above the refractory ledge (1-3″ typically) of circulating Fluidized bed boilers (CFB's). The erosion is specifically seen in certain CFB designs, where a “step” or discontinuity is created at the top of the refractory ledge where the boiler tubes are exposed. The severe wear occurs on the boiler tubes (steel) that extend above the refractory (i.e., above the step), the wear pattern resembling at least one, or generally a pair, of deep “thumb prints” on each boiler tube. These wear marks just above the refractory usually lead to boiler tube leaks, sometimes in periods as short as a few weeks in severe cases (typically these units need to run for one year between outages). This problem is typically addressed by design changes of the refractory/tube interface, or erosion resistant thermal spray coatings in the wear areas. For existing boilers, thermal spray is the most economic method (and typical) of addressing the erosion problem. While coatings offer sufficient protection to prevent tube leaks between outages, they do not generally perform well enough to prevent tube gouging, which requires welding and grinding repair during each outage.
Tube shielding, which may be in the form of weld buildup, wear resistant plate (tube shields), or refractory tiles positioned just above the refractory ledge cannot easily be deployed in an effort to mitigate erosion of the tubes, since the “step” or discontinuity at the top edge of the shielding may become the source of failure.
A technique to prevent tube failure near discontinuities in the boiler is much needed.